1. Field of the Invention
The present invention relates to a rapid dry reagent chemistry test for Helicobacter pylori. More specifically, it relates to a test for determining the presence of a urease producing microorganism such as Helicobacter pylori in human gastric mucosa biopsy specimens.
2. Background Including a Description of the Related Art
In the diagnosis and management of gastrointestinal disorders, the determination of Helicobacter pylori (formerly known as Campylobacter) is becoming increasingly important. The presence of spiral or curved microorganisms in biopsy specimens of human gastric mucosa has been recognized for several decades [Freedberg, A. S. and Barron, L. E., The presence of spirochetes in human gastric mucosa, AM. J. Dis. Dis., 7: 443-5, (1940)]. A bacterium with curved morphology was cultured from gastric-biopsy specimens from patients with gastritis in 1982 [Warren, J. R., Unidentified curved bacilli on gastric epithelium in active chronic gastritis, Lancet, 1:1273, (1983)] and has recently been named Helicobacter pylori. Clinical research since 1982 indicates that H. pylori has a role in causing some forms of gastritis and might also be involved in the pathogenesis of peptic ulcers [Peterson, W. L., Helicobacter pylori and peptic ulcer disease, New Eng. J. Med., 324: 1042-8, (1991)].
H. pylori grows on the gastric epithelium and does not penetrate the tissues. It is also fecund on tissue from the esophagus and duodenum. The bacterium appears to be protected from stomach acid by the action of a urease which the bacterium produces. The urease has very high specific activity [Dunn, B. E., et al., Purification and characterization of urease from Helicobacter pylori, J. Biol. Chem., 265: 9464-9, (1990)] and hydrolyzes endogenous urea from the host tissues to form ammonia which neutralizes stomach acid.
A recent publication suggests that the best means to diagnose H. pylori infections in gastric mucosa biopsies is a combination of culture and direct microscopic observation of the microorganism on tissue following histologic staining (Peterson, W. L., op. cit.). However, the value of culture for this purpose has been questioned [Martinez, E. and Marcos, A., Helicobacter pylori and peptic ulcer disease, Lancet, 325: 737, (1991)].
An alternative test for H. pylori utilizes its urease activity. A mucosal biopsy specimen is incubated in a medium containing urea and a pH sensitive dye [Owen, R. J., et al., Rapid urea hydrolysis by gastric Campylobacters, Lancet, 1: 111, (1985)]. Urease produced by H. pylori releases ammonia through hydrolysis of urea and when enough ammonia is produced to raise the pH of the medium the dye changes color. Human tissues do not produce urease and H. pylori is the principal urease producing microorganism that inhabits the stomach. Occasionally biopsy specimens contain other urease producing bacteria that grow in the assay medium during the incubation and produce a false positive result [Marshall, B. J., et al., Rapid urease tests in the management of Campylobacter pylori]. The urease test for H. pylori relies on preformed urease and cell growth is not required; therefore, the specificity of the test is improved by including a bactericide in the medium. This inhibits the growth of other microorganisms that might contaminate the specimen (loc. cit.).
A commercial test for H. pylori, named CLOtest, described and claimed in U.S. Pat. No. 4,748,113, detects urease activity on biopsy specimens. The incubation medium is solidified by including a gelling agent. This medium also contains phenol red which turns pink when ammonia released from urea raises the pH above 6.0. The system is buffered so that specimens contaminated by fluids from the intestine do not raise the pH and cause a false positive result. When specimens are first inserted into the CLOtest gel they may have a slight pink tinge if blood or alkaline bile is present. The analyst is required to record the initial appearance of the specimen. The test is positive only if the pink color increases in intensity or area.
CLOtest requires three hours incubation at 30.degree. C. and up to 21 hours additional incubation at room temperature. About 75% of biopsy specimens infected with H. pylori give positive results in 20 minutes and 90% are positive by 3 hours. Twenty-four hours are required to verify negative results because 5% of infected specimens become positive between 3 and 24 hours.
The traditional liquid urease tests use a few hundred microliters of medium and ammonia produced by a positive specimen becomes mixed with the medium by diffusion and mechanical stirring. The CLOtest reduces mechanical stirring by using gelled medium and specimens with high urease activity will produce a red color near the specimen in a short time. However, ammonia produced slowly by weakly positive specimens has time to diffuse throughout the gel and a larger amount of ammonia must be produced to give a color change. Thus, the incubation time required for a color change increases disproportionately as the urease content of the specimen decreases. In addition the buffer included in the test medium to consume acid or base on the specimen inhibits pH changes due to production of ammonia and this reduces assay sensitivity and increases incubation times.